Analytical chemistry is the study of matter by means of instruments and methods for separation (isolation of analytes), qualitative identification and quantification. These methods can be used independently or in combination to determine concentration.
Quantification is usually achieved by analyzing the volume or mass changes of the sample. Generally speaking, but not always, a form of separation is required to isolate key components in the sample before accurate quantification can be achieved. Modern instrumentation is used to help improve accuracy, precision, cost, and speed of detection and quantification.
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Separation techniques in analytical chemistry
Separation processes such as chromatography, electrophoresis or field flow fractionation (FFF) techniques enable the reduction in complexity of sample mixtures to isolate individual components for analysis.
Chromatography relies on the mixture being transported in a mobile phase, which can be a gas or liquid and is forced through an immiscible stationary phase. The stationary phase is fixed in place in a column or on a solid surface. The varying constituents within the mixture have differing retention times on the stationary phase.
Thus, the mixture travels at different rates resulting in separation of the constituents within the sample mixture. There are several types of chromatography techniques used in analytical chemistry. Some of these include thin-layer chromatography (TLC), liquid chromatography (LC) and gas chromatography (GC).
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Comparatively, electrophoresis is an alternative separation technique that relies on the motion of particles through an electric field and separates components within the sample mixture based on molecular size, charge and binding affinity.
Additionally, field flow fractionation (FFF) is another separation technique applied to a suspension or solution that separates different components based on their rate of motion as a result of their size and mass.
Traditional methods in analytical chemistry
Qualitative techniques
Traditional qualitative techniques are still being used today in some teaching laboratories. These are usually in the form of chemical or flame tests. A common chemical test is the acid test used for the detection of gold from other base metals.
In contrast, the common flame test identifies metal ions based on their emission spectrum, where the color of the flame is used to identify metal elements.
Quantitative techniques
Quantitative techniques, in contrast, measure the amount of a chemical constituent present in a substance. Common quantitative techniques include gravimetric analysis and volumetric analysis.
A common gravimetric technique is the quantification of water in a sample by weighing the sample before and after heating.
Conversely, a common volumetric technique is an acid-base titration, which involves the addition of a solution until an equivalence point has been reached and a change of color occurs.
Modern methods in analytical chemistry
Modern methods for quantification in analytical chemistry involves the use of instrumentation in the form of spectroscopy, which involves the interaction of light on molecules and matter.
Some common spectroscopic techniques and instrumentation used in analytical chemistry include infrared (IR) spectroscopy, Raman spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, fluorescence spectroscopy, X-ray spectroscopy, atomic absorption spectroscopy, atomic emission spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, photoemission spectroscopy, and Mössbauer spectroscopy.
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Additionally, microscopy techniques use microscopes to view small objects or samples that cannot be seen by the human eye. The common fields of microscopy are optical, electron, scanning probe microscopy and X-ray microscopy.
Optical microscopy and electron microscopy create images by bending the light or electron beams by means of diffraction, reflection, and refraction, to interact with the sample, and then collecting the scattered light or electrons on a detector.
Scanning probe microscopy (such as atomic force microscopy - AFM) involves a surface probe interacting with the topography of the sample. X-ray microscopy is a non-destructive technique that allows for repeated imaging of the same sample, allowing for the ability to see within the sample, which has great applications in biomedical and life science fields.
There are broad applications of optical spectroscopy in biology, chemistry, life science, pharmaceutical, biomedical research, and environmental science. The use of microscopy has been widely used in biomedical research for detecting single cells, single molecules, biological tissue and sub-cellular structures for the early detection and diagnosis of various diseases and cancers.
Conversely, mass spectrometry is commonly used to measure the mass-to-charge ratio of molecules for both pure and complex mixtures using electric and magnetic fields. Combinations of mass spectrometry with chromatography to produce “hyphenated” techniques are widely used for the analysis of complex organic compounds.
Techniques such as GC-MS, LC-IR, LC-MS, and LC-NMR are all used in analytical chemistry to detect and separate constituents in sample mixtures with broad applications in science.
Errors in analytical chemistry
One of the main concerns with using quantification techniques in analytical chemistry is to ensure errors are kept low. An error is the variation of predicted value from an actual value. Typically, it is important to calculate errors in methods, techniques, and instrumentation to give a level of uncertainty attached to the predicted value.
This helps the technician determine the validity of their results to draw statistically relevant conclusions. These defined errors are usually determined by the measurement of standards or calculating statistical metrics such as averages, standard deviations and variances from larger sample size to determine relevant statistical errors.
Analytical chemistry has several applications in a broad range of science disciplines and is mainly driven by accuracy, precision, detection limit, sensitivity and selectivity of techniques. It has a primary focus on performance but is also influenced by the cost of instrumentation, time and speed of the techniques.
References
- Skoog, Douglas A.; West, Donald M.; Holler, F. James; Crouch, Stanley R. (2014). Fundamentals of Analytical Chemistry. Belmont: Brooks/Cole, Cengage Learning. p. 1.
- McMurry, John (2011). Organic chemistry: with biological applications (2nd ed.). Belmont, CA: Brooks/Cole. p. 395.
- Lyklema, J. (1995). Fundamentals of Interface and Colloid Science. 2. p. 3.208.
- Giddings, JC, FJ Yang, and MN Myers. "Flow Field-Flow Fractionation: a versatile new separation method.” Science 193.4259 (1976): 1244–1245.
- Wilkins, C. (1983). "Hyphenated techniques for analysis of complex organic mixtures". Science. 222 (4621): 291–6.
- Ellis, Lyndon A; Roberts, David J (1997). "Chromatographic and hyphenated methods for elemental speciation analysis in environmental media". Journal of Chromatography A. 774 (1–2): 3–19.
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